1. Field of the Invention
This invention relates to a method for producing an optically active carbonyl compound, namely an optically active aldehyde or an optically active ketone, by conducting selective asymmetric hydrogenation of carbon-carbon double bond of an α,β-unsaturated carbonyl compound using a catalyst for asymmetric hydrogenation. Particularly, optically active citronellal can be obtained by conducting selective asymmetric hydrogenation of α,β-unsaturated carbon-carbon double bond of geranial, neral or citral (a mixture of geranial and neral). The optically active citronellal is not only useful by itself as a flavor or fragrance but is also an important raw material of optically active citronellol, optically active isopulegol and optically active menthol.
2. Description of the Related Art
Conventionally, there have been made an attempt for conducting asymmetric hydrogenation of carbon-carbon double bond of α,β-unsaturated aldehyde using hydrogen gas, and there is known a method for conducting asymmetric hydrogenation of neral and geranial for the purpose of obtaining optically active citronellal which is particularly important as a flavor or fragrance (JP-A-54-14911, JP-T-2008-515843 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application)). Since these methods are methods for hydrogenating carbon-carbon double bond with hydrogen gas using a small amount of a homogeneous catalyst, auxiliaries are not required so that a large amount of waste is not generated.
There have been reported an asymmetric hydrogenation of carbon-carbon double bond of α,β-unsaturated ketone using a combination of Pd black, Pd/C or Pd/TiO2 and (−)-dihydroapovincamic acid ethyl ester, proline or cinchonidine (Journal of Molecular Catalysis A: Chemical 1999, 138, 123-127, Journal of Molecular Catalysis A: Chemical 2001, 170, 101-107, Journal of Molecular Catalysis A: Chemical 2002, 179, 101-106, Journal of Molecular Catalysis A: Chemical 2002, 179, 107-112, Journal of Molecular Catalysis A: Chemical 2003, 192, 189-194).
In addition, there has been reported a hydrogen transfer type asymmetric hydrogenation reaction of an α,β-unsaturated compound using an organic asymmetric catalyst and Hantzsch ester (US 2006/0161024, Acc. Chem. Res., 2007, 40, 1327-1339).